Would you please explain to me why it is correct to use "the" before "train"? My teacher told me that "the" is used before objects well-known to both speakers and that if a noun is used for the first time we shoud use "a".

But in this example the speaker uses "train" for the first time. Moreover we don't know about this train, so we must use "a train", not "the train". But it was a native speaker who said it; what exactly did he mean to say?

I know what happened because I was in your shoes before. The trick to understand English articles is to understand that it's all about "definite" (the) vs "indefinite" (a/an). There is a good reason that "the" is called the definite article, not the "second-time" article, nor the "for-something-well-known" article. All the answers below are good. Read them carefully. Once you really understand what "definite" means, you won't be confused easily anymore.
– Damkerng T.Mar 24 '14 at 14:11

4

In this example, the train is obviously "known" in the sense that it's the specific train scheduled to arrive at a specific time. If that wasn't the case, the speaker couldn't know that he had a little time before it was due to arrive.
– FumbleFingersMar 24 '14 at 14:21

Yes, @FumbleFingers has hit the nail on the head :)
– starsplusplusMar 24 '14 at 15:10

Noun being used for the first time is a situation when you would use the definite article, not the (only) situation when you would use it.
– starsplusplusMar 24 '14 at 15:12

3 Answers
3

A better statement of the definite article "rule" would be that you should use the X only when there is a reasonable expectation that your readers or hearers will understand what particular X is meant.

This includes situations when you have previously mentioned X; but it is not necessary to have mentioned a specific train, because few people hang around a railroad station with the plan of taking "a" train, any train that shows up. They're there to catch (or meet) a particular train. Even on a subway system where trains run on any given line every fifteen or twenty minutes, people are waiting for the next train.

Consequently, the hearer or reader may be expected to infer from context that what is meant is “the” train the speaker/writer is waiting for: the train to or from a specific place scheduled to arrive here at this specific time.

"My teacher told me that "the" is used before well-known to both speakers objects and if a noun is used for the first time we shoud use "a"."

The first part of that -- use "the" when the noun is "well-known" -- is basically correct. Most people would say to use "the" when we are referring to a specific instance. Like in this case, the speaker is waiting for a specific train, not just any train.

But the second part -- when used for the first time always use "a" -- is just wrong. There is no such rule.

"The Earth has a population of about 7 billion." Everyone knows which Earth I'm talking about, as there's only one.

"The president of the country gave a speech." It's likely clear from context which country I'm talking about, and any given country probably has only one president. So it's "the", not "a president gave a speech". In context, if I was talking about not just the current president but also about past presidents, I might say, "a president once gave a speech in which he said ...", etc. But if I'm talking about many presidents, then it is not necessarily clear which one I'm talking about at any given moment.

"The wife of the murdered man identified the body." If we said, "A wife of the murdered man ...", that would imply that he has more than one wife, which he probably doesn't.

Etc. There are many cases where the first time that a person or object is mentioned, we would use "the", because we are referring to one specific instance and not one of many.